1. Field of the Invention
The present invention relates to an atomizer for use in producing preforms used to produce optical waveguide fibers either directly or through the intermediate production of a core cane.
2. Technical Background
Signal attenuation is a major factor in the design of communication systems incorporating optical fibers. Transmission losses limit the distance between receivers.
The specific material characteristics which are desired to minimize intrinsic optical attenuation are 1) a large energy gap to minimize absorption in the red and near infrared spectra due to the tail of the ultraviolet absorption band, 2) a low glass transition temperature to minimize density fluctuation scattering and reducing the broadening of the ultraviolet absorption band, and 3) in compound glasses, components with well matched dye element properties to minimize scattering from composition fluctuations. Although fused silica has the beneficial characteristics of a large energy gap and the lack of compositional fluctuations because it is a single component, it has an undesirable high glass transition temperature when compared with many compound glasses.
Many high vapor pressure source compounds that contain elements which provide beneficial properties by decreasing attenuation when incorporated into optical waveguide fibers are exceedingly difficult to make, are excessively expensive, and/or are difficult to handle. These drawbacks make it very difficult to effectively incorporate elements such as alkalis, alkaline-earths and rare earths into the resultant optical waveguide fibers.
As an alternative to employing low vapor pressure compounds, and in order to generate sufficient vapor pressures from the compounds containing the beneficial elements noted above, very high temperatures may be used within an associated burner employed to vaporize such components. However, such elevated temperatures are not compatible with conventional vapor deposition equipment and the production of low loss fibers. An alternative way to deliver low vapor pressure compounds is to spray these compounds directly into the combustion zone in the form of liquid droplets.
In the production of optical waveguide fibers, conventional chemical labor deposition methods such as vapor axial deposition (VAD), modified chemical vapor deposition (MCVD) and outside vapor deposition (OVD) use source compounds, such as chlorides of silicon (SiCl4) and germanium (GeCl4). The source compounds are converted into vapor form using either bubblers or evaporators. The vapor is then transported into a flame and reacted with oxygen to form oxide soot particles. These particles are collected on a rotating starting rod or bait tube in the case of VAD or a rotating mandrel in the case of OVD. In some OVD systems, the cladding portion of the preform is deposited on a previously formed core preform or core cane, rather than a mandrel.
In order for liquid or solution droplets to be converted into solid particles and then deposited on the target, the droplets must evaporate and combust with oxygen to form particles which are then captured on the target. The combustion, size and surface quality of the soot preform are dictated by the particle forming process and capture mechanisms.
Numerous burner designs have been developed for use in such processes, examples of which can be found in Powers U.S. Pat. No. 4,165,223 and Cain et al. U.S. Pat. No. 5,599,371. One of the problems associated with many burner designs is the clogging of the orifices of the face plate of the burners by soot particles. In particular, Suda et al. U.S. Pat. No. 4,801,322 discloses the problem of soot particles adhering to the orifices of a burner.
Another problem often encountered is the clogging of the main orifice of external air-assisted atomizers. External air-assisted atomizers require small exit orifices so that the exiting stream of glass-forming liquid can be effectively sheared by the associated atomizing gas. The relative size of the exiting orifice associated with external air-assisted atomizers as compared to the size of the droplets frequently results in blockage problems of the orifice.
A solution is needed therefore which allows the delivery of low vapor pressure source compounds into a high temperature reaction/combustion zone and converting these compounds into the desired multi-component glass soot, while limiting the adverse effect of soot buildup over and blockage of the orifices of the burner face plate and the exiting orifice of an atomizer associated with the burner system.
One aspect of the present invention is to provide an apparatus for producing a glass soot used in the formation of optical fiber that includes a burner having an internal air-assisted atomizer located within the burner. The atomizer includes an outer tube having a nozzle at an end thereof, and an inner tube located within the outer tube and having a closed end restricting fluid flow therethrough and defining a cylindrical sidewall having a plurality of radially extending apertures. The outer tube receives the glass-forming mixture in liquid form, while the inner tube receives an atomizing gas. The atomizing gas flows through the apertures in the sidewall of the inner tube and atomizes the glass-forming mixture as the glass-forming mixture travels through the outer tube.
Another aspect of the invention is a method for producing a glass soot used in the formation of optical fiber, including providing a burner that includes an internal air-assisted atomizer that includes an outer tube having a droplet-emitting first region, and an inner tube located within the outer tube and having a closed end and a cylindrical wall having a plurality of radially extending apertures. The method also includes supplying a glass-forming mixture to the outer tube of the atomizer, and supplying an atomizing gas to the inner tube, such that the atomizing gas flows through the apertures of the inner tube and enters orthogonally to the flow of the glass-forming mixture within the outer tube, thereby atomizing the glass-forming mixture within the outer tube.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.